The present invention relates generally to a signal processing circuit and method for generating signals representing the division or multiplication of two analog signals. The signal processing circuit and method of the present invention has particular application in the field of optical triangulation distance measurement. More particularly, the present invention has application to photoelectric optical distance measurement utilizing a photoreceiver which generates two analog signals based upon the position of a light beam reflected from a target onto the photoreceiver and wherein the two analog signals contain information relating to the distance from the photoreceiver means to the target.
There may be numerous signal processing applications where either the division or multiplication of two analog signals is desired. The signal processing circuits and methods of the present invention could be applicable in any one of such applications; however, they find particular application in the field of optical triangulation distance sensing.
An illustration of optical triangulation distance measurement system is shown diagrammatically in FIG. 1. A light source comprising a light emitting diode (LED) 10 emits a pulsed light beam that is collimated along its optical axis and directed toward a target or object T. With the target T at the position X1 from LED 10, a portion of the pulsed light beam is reflected back to a photoreceiver means such as a position sensitive detector (PSD) 12, which is also referred to in the art as a lateral effect photodiode. The reflected light beam strikes the surface of the PSD 12 at the position Y1. PSD 12 converts the photon light energy striking its surface into two electrical current signals, I1 and I2 at two of its output terminals. The current signals, I1 and I2, contain information relative to the position where the reflected light beam impinges upon the surface of PSD 12. Accordingly, the current signals I1 and I2 contain information relating to the distance X1. When the position of the reflected light beam moves in a vertical direction, as illustrated in FIG. 1, the difference between the signals I1 and I2 changes. With the target at the position X2, the position of the reflected light beam on PSD 12 is shown at Y2. The displacement between Y1 and Y2 corresponds to the difference between X1 and X2. It is known in the prior art to electronically process the change of the current signals I1 and I2 to generate distance measurement related signals. The position Y of the light beam on the surface of PSD 12 satisfies the following equation:   Y  =            I1      -      I2              I1      +      I2      
In the prior art to accomplish the signal division required to determine the distance X, the most common processing circuitry includes logarithmic operational amplifiers. The current signals I1 and I2 are amplified and converted to voltages V1 and V2, respectively. The division of the two voltages is accomplished with the use of logarithmic conversion according to the following formula       V2    V1    =            log      ⁢              xe2x80x83            ⁢      V2        -          log      ⁢              xe2x80x83            ⁢      V1      
This implementation requires the use of bipolar transistors or diodes in the feedback path of amplifiers resulting in matching, stability and temperature compensation problems. Furthermore, since the relatively inexpensive CMOS processes do not include floating bipolar transistors or diodes, the integration of a log-based architecture would require a large number of external components resulting in a large pin count and consequently large PC board area. This results in a larger minimum package size for the sensor. In general, the integrated circuit (IC) area required to implement log-based architecture in the circuitry for the required signal processing is greater than that in the present invention. The signal processing system and method of the present invention addresses these shortcomings in the prior art. The circuit architecture of the present invention is fully compatible with inexpensive CMOS processes for IC integration. The number of components external to the IC is minimized, leading to smaller packaging requirements.
In one embodiment, the present invention is a signal processing system and method where first and second analog voltage signals are generated. The first analog voltage signal is then used to generate a current signal. A capacitor is charged with the current signal and the voltage on the capacitor is compared with the second analog voltage signal and a signal is then generated having a time interval that represents the division of the first and second analog voltage signals. In another embodiment, the signal processing circuit and method generates first and second analog voltage signals. The first analog voltage signal is used to generate a first current signal and the second analog voltage signal generates a second current signal. A first capacitor is charged with the first current signal and the voltage on that capacitor is compared with a predetermined reference voltage and an output signal having a time interval representing the time required to charge the first capacitor to a voltage exceeding the reference voltage is generated. That output signal controls the charging of a second capacitor with the second current signal in the time interval whereby the voltage on the second capacitor then represents the multiplication of the first and second analog voltage signals.
The present invention is based upon the following principle. The current I required to charge a capacitor C to a voltage xcex94V in a time xcex94T, is defined as:
I=C(xcex94V/xcex94T)
This formula can be rewritten as:
T=C(xcex94V/I)
With I=V1/R, where V1 is the magnitude of a continuous input voltage V1 sampled at time tS and with xcex94V=V2, where V2 is the magnitude of a continuous input voltage V2 sampled at time tS, then the following formula applies:
xcex94T=Cxc3x97Rxc3x97(V2/V1)
Thus, xcex94T is the result of the division of V2/V1 multiplied by a constant. This result is valid for any system where the maximum rate of change of V1 and V2 is less than the minimum xcex94T required to represent this change. The result, xcex94T, can easily be converted into an analog voltage level for standard analog signal processing, or it can be used in its current form for a wide range of digital signal processing. A similar calculation leads to the result that xcex94V=A(V1xc3x97V2) where xcex94V is the voltage to which the capacitor is charged in the interval xcex94T and represents the multiplication of the two voltages V1 and V2 with a constant multiplying factor A.
In the application of the signal processing system and method of the present invention to optical triangulation distance measurement, the two analog signals are voltage signals representing the currents I1 and I2 from the PSD. The two analog voltage signals are pulsed signals which are then converted to DC signals by sample and hold circuits which sample each voltage signal with the occurrence of each light pulse from the light emitting diode of the system. One of the two analog DC voltage signals is converted to a current signal which is used to charge a divider capacitor. A divider comparator has one input connected to the capacitor and a second input connected to the other of the DC analog voltage signals. The output of the comparator is a signal having a time interval that represents the division of the first and second analog voltage signals. The system has a range adjustment circuit that generates a signal representing the sensor range. A decoder has an input connected to the output of the divider comparator and a second input receiving the range signal from the range adjustment circuit. The decoder then generates an output signal that represents whether or not the target is within the preset range of the system. The divider capacitor is discharged with the occurrence of each light pulse of the light emitting diode. In this embodiment the distance to the target that is a function of the subtraction of the two analog voltage signals divided by the sum of the two signals is simplified to a division of the two analog voltage signals without significant loss in accuracy.
The present invention can be applied to optical triangulation and distance measurement by inexpensive CMOS processes for an application specific integrated circuit (ASIC). The present invention minimizes the requirements for components external to the integrated circuit, thereby also minimizing the package size requirements. The present invention also has an advantage over the prior art of a wide dynamic range, temperature stability, and high noise immunity. The present invention also eliminates the need for bipolar matching requirements in circuitry and provides gain stability under varying signal conditions. The present invention has low power requirements and wide range adjustment capability. These and other advantages of the present invention will become apparent with reference to the accompanying drawings, detailed description of the preferred embodiments, and the claims.